Initial sizes typically ranged in capacity from around 200 to 10,000 litres, or multiples of these. The most popular sizes are 2500 litres and 5000 litres, which are easily handled and are not unsightly.
Smaller tanks, such as the plastic 200-liter rainwater tanks are also used in some cases. These smaller tanks are mainly used in conjunction with a Garden Rhapsody Grey Water Re-using System. With this arrangement water tanks fitted above ground may flow by gravity into the Garden Rhapsody which has a pump incorporated into it and will pump the rain / grey water onto the garden. This can even be done automatically by trickle feeding the water from the higher rain tank/s into the Garden Rhapsody at all times, so that rain water flowing into the rain tank/s will not fill a tank that is full already, no matter how small the tank is.

Larger tanks are commonly used where there is no access to a municipal water supply. In this instance at no extra charge, Water Rhapsody will recommend a tank at a house taking into consideration a whole host of factors like: roof type surface (tile or metal), roof area in square metres, number of people drawing water from the rainwater tank, whether or not one has the rest of the Water Rhapsody systems in place to reduce ones consumption, and whether the house is a home, holiday home or a commercial or industrial building.

To give an example of this calculation, (this was done by actuaries for Water Rhapsody): a typical metal roof will deliver 1000 litres of water from 100 square metres of roof for every 11 mm of rain. To get the same volume of water from a tiled roof one would need 16 mm of rain. Each middle class person uses 240 litres of water daily, so the draw from the rain tanks is very substantial. If one should install only two of the Water Rhapsody Systems to say: re-use grey water and the Water Rhapsody Multi-Flush to minimize toilet flushing, one reduces the demand for water to at worst half (of the 240 litres to 120 litres per person per day), but mostly down to as little as to 80 to 100 litres per person per day. Stored Rainwater then goes so much further without a change in lifestyle. The value of this calculation ensures that there is enough storage so that rainwater tanks need never overflow, and that one does not overspend on too many rainwater tanks that never fill.